add min and max options to DetectorScene
E.g. in matplotlib's imshow, the image is transformed as
intensity -> (intensity - min) / (max-min)
where max and min are the maximum and minimum intensities of the visible frame. This is very convenient for visualizing weak peaks in protein crystal data.
In contrast, in DetectorScene we scale the image as intensity -> intensity / max
, where max is a user-defined value. Max being user-defined is not so bad, but without also subtracting a minimum value, it makes it very difficult to visualize weak peaks.